This site has long shared the Government’s view that the best chance of the Brexit talks producing a good deal rests on making absolutely clear that the alternative is leaving the EU without a deal. Those who criticise the idea that “no deal is better than a bad deal” somehow fail to see that sending a message to Brussels that Britain will settle for anything – as the Labour manifesto suggested – is a fast way to ensure they offer us very little, at an extremely high cost. No negotiation that begins with one party telling the other “name your price, I’ll buy regardless” ends well for the customer.

The crucial thing about threatening to walk away from a negotiation is that it must sound – and be – credible. David Cameron occasionally tried, somewhat lamely, to suggest that if the EU renegotiation failed then he might back Leave in the ensuing referendum. But the EU did not believe him, not least because he could never even bring himself to say the words out loud, only managing “nothing is off the table” as his most drastic threat. His negotiating partners saw straight through that line, and as a result he was only offered a pale imitation of the already very limited things which he had asked for. His negotiation failed, he backed Remain anyway, and lost. Had he banged the table and walked out, he might have got a better offer and history might be different – but Brussels judged that he would not do so, and they were right,

So it is essential that the Government demonstrate that the prospect of leaving without a deal is believable and believed. On that measure, the Prime Minister’s statement yesterday and the accompanying white papers were very welcome. Contingency planning to manage the technicalities and practicalities of trade on WTO rules is underway and, if further progress has not been made by the New Year, spending will begin on the computer systems and infrastructure needed to implement it. That’s the responsible thing to do for the country, and the wise thing to do for the negotiations.

However, the Prime Minister’s statement was more troubling on the question of how she envisages the legal regime in the hoped-for transition period after March 2019. It seems that the Government still plans for any deal to be ultimately overseen by a new arbitration court, potentially some form of sub-tribunal hosted by the EFTA court as floated over the summer. But yesterday May suggested that the move to such a system would take place during the transition period, not at the start of it – effectively delaying the point at which Britain escapes rule by the European Court of Justice to some point between 2019 and 2021.

This is dangerous territory. For a start, many of the 17.4 million Leave voters are naturally concerned that, having finally won their victory, the politicians in Westminster will seek to rob them of it through sleight of hand. Given that “our laws” was one of the holy trinity of things the Leave campaign said it wanted to “take back control of” (along with “our borders” and “our money”), ending the ECJ’s power over British law is highly important to most Eurosceptics. Indeed, in our recent survey of Conservative Party members it came second only to the freedom to strike new trade deals in terms of things people wanted to see happen during the implementation period – a full 19 percentage points more popular than reducing immigration. For many – me included – regaining domestic, democratic control of our laws is the single most important prize of Brexit.

The risk is not only that this phased withdrawal risks alienating Leave voters; it could also embolden those Remainers who still hope to blunt or even prevent Brexit altogether. The history of European integration is littered not only with the bodies of Conservative politicians, but also with the very persistent eyesores of supposedly temporary arrangements. If the ECJ is allowed to retain its power after Brexit Day in March 2019, even temporarily, some will conclude that it might be possible to retain it indefinitely, or even permanently. The Continuity Remain campaign has largely been an exercise in ill-advised fancy dress, and wild rhetoric from previously respected public figures, but there is no sense in offering it any encouragement in its misguided belief that there might be some technicality which offers a way to ignore the referendum result.

The Government’s attempt to stave off the concerns of Leavers and dampen the hopes of Remainers is to say that this would be a temporary arrangement. As the Prime Minister put it, “we have to negotiate what will operate during that implementation period…that may be that we will start off with the ECJ still governing rules for part of that period.” That might well be true, but it is neither reassuring enough to those supporters who are concerned nor clear-cut enough to those opponents who are hopeful. As a bare minimum, there must be far greater clarity about the timescale being considered, and the mechanism to ensure that it is held to. Even then, all of us who have witnessed the EU’s tendency to simply ignore supposedly concrete agreements are unlikely to find it very soothing.

The same goes for the even weaker answer to concerns that during such a period Britain would be subject not only to existing EU law but to new laws, too. Simply saying that it is “highly unlikely” that Brussels will legislate during that time is not good enough. If we’ve learned anything from the last four decades, it’s that the EU does what it wants regardless of public opinion, very often to our detriment. The vote to leave was a pretty clear message that voters are no longer willing to simply trust in the beneficence of the EU institutions.

It’s interesting to note how this is playing in May’s own Parliamentary Party. Various Eurosceptic backbenchers are reported to be unhappy about the prospect of staying under ECJ jurisdiction for a minute longer than we are EU members. But Boris Johnson and Michael Gove, key Brexiteers that they are, have publicly endorsed the Prime Minister’s message.

This is a breach of Johnson’s latest ‘red lines’, which supposedly included not following new EU laws or ECJ rulings, and not shadowing existing EU law during the transition. May appears to have persuaded him or called his bluff. Gove’s tweet casts this as a question of being “pragmatic” to secure the end goal of a proper Brexit. Both men chose to emphasise that they believe such a transition plan is necessary and acceptable to secure the “end goal”, free of the ECJ.

Of course, there’s one caveat to all this: a transition period will only happen if there’s a deal. If the Brexit talks produce an agreement on the future relationship, they will also settle the question of how to shift from membership to that new arrangement. If there’s no deal, then there’s no transition, which is all the more reason to ensure Britain is ready to operate fully on day one.